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Petrography,geochemistry and geochronology of the host porphyries and associated alteration at the Tuwu Cu deposit,NW China: a case for increased depositional efficiency by reaction with mafic hostrock? |
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| Authors: | Ping Shen Hongdi Pan Taofa Zhou Jingbin Wang |
| Institution: | 1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing, 100029, China 2. College of Earth Sciences, Chang’an University, Xi’an, 710054, China 3. School of Resources and Environment Engineering, Hefei University of Technology, Hefei, 230009, China 4. China Non-ferrous Metals Resource Geological Survey, Beijing, 100012, China |
| Abstract: | Tuwu is the largest porphyry copper deposit discovered in the Eastern Tianshan Mountains, Xinjiang, China. A newly recognized volcanic complex in the Early Carboniferous Qi’eshan Group at Tuwu consists of basalt, andesite, and diorite porphyry. The plagiogranite porphyry was emplaced into this complex at 332.8±2.5 Ma (U–Pb zircon SIMS determination). Whole-rock element geochemistry shows that the volcanic complex and plagiogranite porphyry formed in the same island arc, although the complex was derived by partial melting of the mantle wedge and the plagiogranite porphyry by partial melting of a subducting slab. The diorite and the plagiogranite porphyries have both been subjected to intense hydrothermal alteration and associated mineralization, but the productive porphyry is the plagiogranite porphyry. Three alteration and mineralization stages, including pre-, syn- and post-ore stages, have been recognized. The pre-ore stage formed a barren propylitic alteration which is widespread in the volcanic complex. The syn-ore stage is divided into three sub-stages: Stage 1 is characterized by potassic alteration with chalcopyrite + bornite + chalcocite; Stage 2 is marked by chlorite–sericite–albite alteration with chalcopyrite ± pyrite ± bornite; Stage 3 is represented by phyllic alteration with chalcopyrite + pyrite ± molybdenite. The post-ore stage produced a barren argillic alteration limited to the diorite porphyry. A specific feature of the Tuwu deposit is that the productive porphyry was emplaced into a very mafic package, and reaction of the resulting fluids with the ferrous iron-rich hostrocks was a likely reason that Tuwu is the largest porphyry in the district. |
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